Photoconductive insulating material



April SURFACE POTENTIAL (VOLTS) 1970 KATSUO MAKINO ETAL PHQTOCONDUCTIVE INSULATING MATERIAL Filed Oct. 31, .1966

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0 50 |00 me 200 250 TIME PASSED AFTER CHARGING (Sec) 0-01 BEFORE-EXPOSURE |o00- 1 /AN| BEFORE EXPOSURE.

A-ll3 AFTER EXPOSURE 0 A- m AFTER EXPOSURE I mvsm'dns 0 30 msuo muuuo lwAo- SAWATO A1 4, Fol/Ma? flu, M ATTORNEYS United States Patent U.S. c1.2s2--s01 3 Claims ABSTRACT OF THE DISCLOSURE A photoconductive insulating material comprising cadmium sulfide and cadmium carbonate photoconductive powder having metal iodide incorporated therewith, the molar ratio of the metal iodide to the cadmium sulfide and cadmium carbonate being approximately 0.01 to 0.1. The powder is'dispersed through an electrically insulating binder.

This invention relates in general to a photoconductive insulatingv material and more particularly to an electrophotographic photosensitive member having high photosensitivity comprising photoconductive fine powders dispersed in a binder.

The photoconductive insulating member is herein defined as a member the electrical resistance of which is higher than ohm-cm. in a place where no radiation exists, but the resistance of which is decreased when it is irradiated by light or other radiations.

In electrophotography, various kinds of processes have been conducted, but brief explanations will be given hereafter on the most widely known and employed process for convenience and for a better understanding of the subject matter of this invention. An electrophotographic photosensitive member is first electrostatically charged by corona discharge in a dark place. The charged member is then. exposed to a light image to be reproduced by a conventional exposing method-The charges leak off in proportion to the intensity of light to which any given area is exposed. After such exposure, a latent electrostatic image is formed on the surface of the member. Development of the latent image is carried out by contacting the member bearing the latent electrostatic image with electrostatic marking particles. These particles adhere to the areas where the electrostatic charges remain, forming a powder image corresponding to the electrostatic image. The powderdmage can then be fixed-directly onto the surface of the photosensitive member or transferred to a sheet of transfer material and fixed thereon by a suitable method. In the case of transferring the powder image, the residual powder on the surface of the photosensitive member is cleaned off and the member is used repeatedly. The process described above is the most popular one. Though various kinds of electrophotographic processes have been employed besides the aforesaid process, the essential properties of the photoconductive insulating layer of the photosensitive. member are substantially. identical in such processes.

The potential acceptance? is hereinafter defined to be the surface potential of an electrophotographic photosensitive member when the member is charged electrostatically by, for instance, corona discharge; and the half decay exposure time isdefined to be the period of time required for the, surface potential to reduce to half of the initial potential during light exposure. An electro photographicphotosensitive member of higher potential acceptance and of the shorter half decay exposure time is desirable. These properties aredeteriorated in general when the photosensitivemember isjirradiated with intense 3,506,595 Patented Apr. 14, 1970 light before being charged. In particular the potential acceptance becomes lower. This is the so-called light fatigue or pre-exposure effect. A higher potential acceptance generally corresponds to a higher dark resistance of a photoconductive member, although there does not exist a clear one-to-one correspondence between them. The sensitivity of the photocurrent of a photoconductive member corresponds to the half decay exposure time thereof. We, the inventors, have found that fine particles of a cadmium sulfide-cadmium carbonate photoconductor are excellent electrophotographic photosensitive material. The photoconductive insulating member having the above-mentioned powders can be charged in positive or negative, by a suitable corona discharging and the like, and shows substantially the same potential acceptance as well as substantially the same photosensitivity (exactly speaking, a little higher sensitivity in the negative polarity than that in the positive polarity) to both polarities. These powders are, however, inferior to vitreous selenium in photosensitivity and light fatigue, as described hereinafter, and are insufiiciently used in place of the vitreous selenium, and hence is required that their properties be improved.

Hereupon, in an automatic copying machine a higher photosensitivity is imposed upon an electrophotographic photosensitive member. Further, electrophotographic members to be used in copying machines where a transfer process is used are required to have sufficient electrophotographic properties and mechanical strength to endure repeated use. The inventors have studied electrophotographic photosensitive members mainly consisting of a photoconductive powder of cadmium sulfide, and of a cadmium sulfide-cadmium carbonate photoconductive powder dispersed in a binder as an electrophotographic photosensitive member to be used repeatedly in an automatic copying machine, but their properties are found to be inferior to a photosensitive member mainly consisting of vitreous selenium. The study has further been carried out in order to improve their properties and the incorporation of iodine has been found to be effective to satisfy the requirements. Namely, by adding iodine, a photosensitive member comparable to the vitreous selenium photosensitive member for the repeated use in an automatic copying machine has been obtained. However, in the case of adding elementary iodine there are some difiiculties in the production of members having stable properties and constant composition, since some amount of iodine will be lost during the addition or drying processes owing to its high vapour pressure. v I

In order to overcome the above difficulty in the manufacturing process, the inventors have studied a method where iodine is added as a form of a metal iodide and have obtained satisfactory results. Therefore, this invention relates to the photoconductive insulating material which is produced by dispersing in an'electrically insulating binder the photoconductive powders mainly com: prising a cadmium sulfide-cadmium carbonate photoconductor having a form of, for example,

together with a metal iodide. The metal iodides used in the invention may be water soluble and/or organic solvent soluble. ones. For examples, there are lithium iodide, magnesium iodide, beryllium iodide, bismuth iodide, tungsten iodide, cesium.iodide, strontium iodide, tin iodide, potassium iodide, cad-v mum iodide, antimony iodide, aluminumtiodideand zinc iodide. These compounds may be incorporated in the cadmium sulfide-cadmium carbonate photoconductive powder as a solution. thereof, i.e-., the powder may: be impregnated with the metal-iodide solution.and"dried. In another way, the photoconductive powder may be treated with the metal iodide vapour to adsorb it or diffuse it into the powder particles. A metal iodide which cannot be dissolved into water or organic solvents, such as lead iodide, may be also employed in this invention and, in such a case, the iodide may be incorporated in the photoconductive powder by heating the powder in the vapour of the iodide formed by heating it.

The cadmium sulfide-cadmium carbonate photoconductive powder used in the present invention may be prepared by mixing an aqueous solution containing a carbonate ion and a sulfur ion with an aqueous solution containing a cadmium ion, or by mixing an aqueous solution containing a carbonate ion with an aqueous solution containing a cadmium ion in the presence of hydrogen sulfide, and, thereafter, baking the thus precipitated reaction product. As the cadmium salt there may be used a cadmium halide, cadmium sulfate, cadmium nitrate and the like and, as the carbonate, there may be used sodium carbonate, potassium carbonate, ammonium carbonate and the like. The cadmium sulfide-cadmium carbonate photoconductive material used in this invention is not a simple mechanical mixture of cadmium sulfide particles and cadmium carbonate particles, but it is considered that one particle of the powder consists of both cadmium sulfide and cadmium carbonate although its particle structure and/or its crystal structure have not yet been understood.

In X-ray diffraction analysis of the photoconductive material the diffraction lines of cadmium sulfide and cadmium carbonate are observed independently. But their diffraction lines are broad and diffuse, which indicates the crystallinity being not so high. The ratio of both components is preferably that n, in the form of CdS-nCdCO is less than 4.

The binder used in the invention should be electrically insulative and should not be chemically, electrically and physically poisonous to the photoconductive powder. For example, a synthetic resinous binder is preferable. And as the resinous binder, there are room temperature setting types, catalytically setting types, and thermosetting types of resinous binders, and they may be used in accordance with its purpose of usage. Various combinations of the above binder materials may be also employed. Further, as the binder, a low melting glass (a kind of inorganic polymer) may also be used.

When elementary iodine is added to the photoconductive powder, it is required that special care be given to the atmospheric conditions, in drying and heat treatment processes, for preventing the iodine from being volatilized off, but such a care is not needed when using metal iodide compounds in accordance with the present invention. The photoconductive powder incorporated with the metal iodide is dispersed in a binder to form a photoconductive insulating member, whose properties are similar and comparable to the member incorporated with iodine as well as to the member of vitreous selenium. The photosensitivity of the member thus prepared becomes several times higher and the light fatigue thereof decreases in comparison to the member without any iodine and metal iodide. Thoughexplanations are herein given mainly about a layer form photoconductive insulating member, other forms of the member may be presented. In- 'stead of coating the dispersion of the photoconductive powder in the binder on a support, such as a metal plate or a paper sheet, the dispersion may be cured and ground into comparatively large particles, ranging from several microns to several hundred microns, which may be used, as it is, 'as a photoelectric photosensitive material.

'Moreover, the electrophotographic photosensitive material of the invention is superior for the photosensitive member on which the powder image is directly fixed.

Further, the photoconductive insulating member of the invention can be used not only for the photosensitive material in various kinds of electrophotographic processes, but also for the photosensitive material in various kinds of photoelectric converting processes. For instance, it may be used for the photosensitive material in a photocell, a light image convertor, such as a light image amplifier and a vidicon, a phototape or X-ray-to-visible image convertor, etc.

In this specification, explanations have been given mainly about the photoconductive insulating member which is used as the electrophotographic photosensitive member, but the applications of the invention should not be restricted within these explanations and illustrations.

The following examples are now presented as detailed illustrations.

Examples 14 In accordance with the prescriptions described in Table 1, shown below, 450 mls. of an ethyl alcohol solution of cadmium iodide was mixed with a cadmium sulfide-cadmium carbonate powder of CdS-1.5CdCO That is, the powder was dispersed in pure ethyl alcohol, into which an ethyl alcohol solution of cadmium iodide was added dropwise with agitating. The slurry, thus obtained, was then well agitated and dried at a temperature of C. to C. for 24 hours. About half of each dried powder in Example 1 or Example 2 was further baked in a glass bottle in an air oven at a temperature of 200 C. for 24 hours to prepare the powders of Example 3 or Example 4, respectively. Thereafter, in accordance with the prescription described in Table 1, each powder thus treated was mixed and dispersed in a binder in a porcelain ball mill to make a photosensitive paint, the binder used herein being a thermosetting acrylic resinous binder named Magicron No. 200 Clear manufactured by Kansai Paint Co., Ltd. From the thus prepared photosensitive paint electrographic photosensitive members having various thicknesses were prepared by coating on aluminum sheets, drying them and baking them at the temperatures of C., 200 C., and 250 C.

The measurements of the potential acceptance and the half-decay exposure time of the electrophotographic photosensitive members thus obtained were carried out by charging them by 7.0 kilovolts corona discharge. The potential acceptance and the half-decay exposure time were increased as the thickness of the photosensitive layer was increased. Therefore, the relation of the potential acceptance to the half-decay exposure time of each of the photosensitive members which were made of the same photosensitive paint and by the same baking condition formed one relation curve. From the curves thus obtained from the various kinds of photosensitive paints and by the various kinds of baking conditions, the half-decay exposure times which correspond to the potential acceptance of 500 volts were estimated. The half-decay exposure times thus obtained for various kinds of the photosensitive members are listed in Table 1 as S-HDET. The baking conditions A, B, and C, in Table 1 denote the baking temperatures and the baking times, i.e., at 150 C. for 30 min., at 200 C., for 60 min., and at 250 C. for 30 min., respectively. By these examples the fact that the photosensitivity increased with the cadmium iodide concentration was confirmed, the photosensitivity being considered to correspond to the reciprocal of the half decay exposure time. In Examples 1 and 3, where the quantity of iodide added was small, the photosensitivity increased as the baking temperature became higher. On the contrary, in Examples 2 and 4, where large quantities of cadmium iodide were added, the photosensitivity decreased as the baking temperature increased. This tendency is evident in Example 4, where the powder incorporated with cadmium iodide was fired. Anyhow, the highest photosensitivity was reached at the baking condition A in Example 4, which can be thought to indicate that sufiicicnt photosensitivity can be obtained, even if there is no baking of the coating for curing of the binder material. In the case of charging in a positive polarity, FIGURE 2, the number of repetitions of use is plotted the same tendency as wlth char'g1ng' 1n a negative polarity on the abscissa and the surface potential on the ordinate. was observed, though the' ph'otosensitivity was "a little Excellent characteristics have been obtained. Further copy lower, as listed in Table 1. samples were actually taken by the above conditions p TABLE 1 [Relation between the preparation conditions of sample in Examples 1-4 and S-HDET] s-HDET Binder Cdlx M01 7 (Solid (-7.0 kv.) (sec.) (+7.0 kv.). (sec.) CdS'L5CdCO percent Part) Powder (grams) added Firing condition (grams) A B O A I: B

Example 1 60 1.0 i 30 3.1 2.4 2.0 3.0 2. 45 2.6 Example 2-". 60 10. 0 30 1. 65 1. 4 2.1 2. 55 21. 5 4. 2 Example 3-.-- 60 a 1. 0 200 0., 24 hours 30 2. 2 1. 9 1.6 2. 0 2. 0 2. 5 Example 4 60 10.0 .d0 30 3 0 1.3 1.65 1.5 1. 45 3.0

S-HDET-Halt-decay exposure time of the sample which shows the potential acceptance of 500 volts.

FIGURE 1 shows the changes of surface potential in resulting in good copies with high image contrast and a dark place of the photosensitive members of the exlow background density.

amples in the invention and of the others for comparison. I

On the ordinate is plotted the surface potential in the Examples 7-20 logarithmic scale and, on the abscissa, the period" of time V i ki d of t l i did were dd d t h h passed after charging. Sample N0. 4434 is a representato conductive powder comprising cadmium sulfide and tive of Example 2. Sample No. 4470 represents Example carbonate CdS-1.5CdCO as alcohol solutions or aque- 4. Sample No. 2384 is a representative of the photoous solutions, in the same way as in the Examples 1-4 to sensitive member of the powder which is not incorporated prepare the photoconductive powder incorporated with with cadmium iodide. All preparation conditions of metal iodide in accordance with the prescription described Sample No. 2384 are the same as those of the sample in Table 2. Then the photosensitive paints were prepared incorporated with cadmium iodide, except for incorporaby dispersing the powder thus obtained in a binder tion of iodide. In FIGURE 1, the curves noted with the material with porcelain ball mills in accordance with mark 0 show the surface potential changes in the dark the prescription described inthe Table 2. The binder measured at the rest state (after placed in the dark place material used herein was a kind of thermo'setting acrylic for more than 24 hours), the curves noted with the resinous binder named Magicron No. 200 Clear manumark O show those measured at 300 seconds, after prefactured by Kansai Paint Co., Ltd. In the same Way as exposure to 2000 lux for 2 minutes, and the curves with those described in Examples 1-4, the preparation of the the mark A show those measured at 15 seconds, after photosensitive member for electrophotography, and the pre-exposure to 2000 lux for 2 minutes. From this measurements of their characteristics were carried out. figure it becomes clarified that Sample No. 2384 for The results are listed up in Table 2. Some samples in comparison can hardly be charged to be measured in the examples showed the decreasing half-decay exposure the condition noted with the mark A, shows little potime wlth the increasing thickness, though the potential tential acceptance only in the condition noted with the acceptance increased with the increasing thickness of the mark 0, while the samples in the examples show rephotosensitive layer.

TABLE 2 S-HDET 1 Metal Halides Added Ratio of (-7. 0 kv.) (sec.) (+7.0 kv.) (sec).

Halide Grams Firing conditions Powder A4 B C A B (3 Example 7 ZnIz 1/2 2.10 1.8 1.45 2. 6 256 3.1 Example 8... Znle 1/2 0.50 0.43 0.50 1.5 1 .5 1.5 Example 9--. A11 1/2 2.8 2.0 1.65 3.2 2.2 1.9 Example 10.. A113 0- 40 0.52 l. 3 3.4 Example 11 B11 8. 1/2 2.9 2.9 2.5 3.0 2.3 2 5 Example 12 BiIa 8. 5 0.50 1.8 1. 6 Example 13 SbIa 7. 1/2 2. 35 2.05 1 5 3.0 3.0 2.0 Example 14. SM; 7. /2 0. 65 0.60 2. 8 3. s Example 15 KI 2. 1/2 2. 05 2. 0 1. 5 2.8 3.2 2 0 Example 16 K1 2. 1/2 0. 92 0. 5 0.6 1. 7 1. 75 1. 7 Example 17 HgIz 6. 1/2 a. 05 2.8 2. a 4. 1 3. 6 3. 3 Example 18-. HgIz 6. 200 0., 24 hours- 1/2 0.73 0 0. 55 1. 5 1. 35 1.3 Example 19 LiI-nHzO 7. 5 1/2 1.8 1. 3 0.89 2. o Example 20 LlI-nHzo 7.5 200 0., 24 hours 1/2 0.3 0.75 0.58 1.55 1.2 1 45 1 Half-decay exposure time of the electrophotographic photosensitive member which shows the potential acceptance of 500 volts. 2 Metal halide was added to 242 grams of the powder. 3 Binder material was mixed with the powder incorporated wlth metal hahde at the weight ratio described. 4 The baking conditions of the coatings A: 150 0., 30 min.; B: 200 0., min.; C: 250 C., 30 min. markable improvement, though they have a little light 60 EXAMPLE 21 fatigue effect Stln- In 100 grams of a binder material of a kind of thermo- EXamP1eS5and6 settirlg acrylic resin called Magicron No. 200 Clear Two photosensitve drums were prepared by coating (solid part being about 50%) and 120 1111s. of thinner, aluminum tubes with the photosensitive paint of Ex- 3.1"grams of aluminum iodide, All was dissolved. Then, ample 4, drying the coating at 70 C. for 50 minutes and in this binder containing A11 there was dispersed 100 baking them at 150 C. for 30 minutes. The coating thickgrams of powder comprising CdS-CdCO to provide a nesses of the photosensitive layers of the drums were photosensitive paint, which was coated on aluminum about 3.2 mg./cm. for Sample No. A-111 in Example sheets, dried at C. and then bakedat 200 C. for 30 5 and about 6.13 mg/cm. for Sample No. A-113 in minutes to form photosensitive members.

Example 6 after baking. The surface potentials before 70 The electrophotographic characteristics of the photoand after exposing were measured and their changes sensitive members thus obtained were evaluated in the during repeated use were examined by charging with same manner as described in the Examples 1-4, result- -6.5 kv. corona discharge and exposing them to about ing -in photosensitivity about 2.5 times as high as those 100 lux. sec. as they were rotated at a revolution rate of the photosensitive member not containing aluminum of 6 r.p.m. The results are shown in the FIGURE 2. In iodide.

What is claimed is: r 1. A photoconductive insulating material consisting essentially of photoconductive powder particles and an electrically insulating binder selected from the group consisting of organic resinous binders and low melting point glasses in which the said photoconductive powder particles are dispersed, said powder particles consisting mainly of cadmium sulfide-cadmium carbonate photoconductive material (that is, CdS-nCdCO n 4) having a metal iodide incorporated therewith, said metal iodide being selected from the group consisting of water soluble iodides,

organic solvent soluble iodides, and iodides which form a vapor upon heating and where the molar ratio of said metal iodide to said cadmium sulfide-cadmium carbonate photoconductive material extends from approximately 0.01 to 0.1.

2. A material in claim 1 where n is 1.5 and the said molar ratio is 0.1.

and lithium.

References Cited UNITED STATES PATENTS 2,866,878 12/1958 Briggs et a1. 2.52-501 3,324,299 6/1967 Schuil 252--501 3,345,161 10/,1967 Marnmino et a1 96-15 MAYER WEINBLATT, Primary Examiner W. E. SCHULZ, Assistant Examiner US. Cl. X.R. 961.5 

